In veterinary medicine, few diseases seem to break the hearts of veterinary professionals more often than injection-site sarcomas in cats. This is because these cancerous tumors are believed to be caused by an inflammatory reaction to a simple injection that was originally given to help the cat live a longer, healthier life – but sadly, ended up having the opposite effect.

Sarcomas are malignant tumors that begin within the body’s connective tissue, which is a specialized type of tissue whose job it is to support, bind, or separate other tissue and organs. Examples of connective tissue include fascia, bone, cartilage, tendons, and fat. Most injection-site sarcomas in cats are fibrosarcomas, a highly aggressive form of feline cancer. These tumors can spread rapidly through surrounding tissue and throughout the rest of the body.

Fortunately, injection-site sarcomas are rare. Depending on the source, statistics show that they are reported anywhere from one in 1000 to one in 10,000 cats. Although most veterinarians will only see a small number of these tumors during their career, they do occur, and for cat parents, it’s important to be aware of them and know how to recognize them quickly.

What Are Injection-site Sarcomas?

Injection-site sarcomas in cats were first diagnosed around 1991, when the School of Veterinary Medicine at the University of Pennsylvania discovered a link between an increase in sarcomas and certain types of feline vaccinations. Although these types of reactions can occur in other animals, cats seem to have a more extreme reaction, so it’s far more common to see these tumors in cats.

An injection-site sarcoma (also referred to as “ISS”) is a specific type of tumor that appears at the site of a previous injection – that is, a substance that was injected directly under the skin or into muscle tissue through a needle. Reported causes of ISS have included long-acting antibiotics, certain types of steroids, injectable Lufenuron (the active ingredient in some flea and tick medications), and microchips. However, the most common culprit when it comes to injection-site sarcomas seems to be vaccines.

Why Cats Need Vaccines

Before we talk about vaccines and how they relate to injection-site sarcomas, first please understand that cats (especially kittens, whose fragile immune systems are not yet developed, making them very susceptible to illness), absolutely need initial vaccines. Vaccines are critical in helping a cat’s immune system learn how to fight infections from microorganisms that can cause deadly diseases.

All kittens need to be vaccinated, but once cats reach adulthood, they generally only need to be re-vaccinated once every few years.

Vaccines contain either weakened or killed microorganisms that stimulate a cat’s immune system to produce special proteins called antibodies. Afterwards, if the cat encounters that microorganism in the real world, the antibodies will help defend against an infection. Vaccines help the cat’s immune system to recognize these microorganisms quickly and fight them much more efficiently, which can lessen the severity of the infection or prevent it altogether.

The Link Between Vaccines And Injection-site Sarcomas

Although for the vast majority of patients vaccines pose an extremely low risk, it is true that no vaccine is completely risk-free. And research indicates that unfortunately, sarcomas that form as a result of vaccines seem to be more aggressive than non-vaccine sarcomas.

It’s believed that the reason vaccines are most implicated in the development of injection-site sarcomas is because vaccines are purposely designed to cause temporary inflammation when they’re injected. This inflammation helps “kick-start” the body’s immune system to form antibodies.

However, many vaccines contain additional substances called adjuvants. These are ingredients that are added to vaccines to help hold the vaccine in the tissue around the injection site for a certain period of time in order to allow more time for the immune system to be properly stimulated. However, in some cats this inflammatory response goes into overdrive, triggering the formation of an injection-site sarcoma.

The most common vaccines linked to ISS are the ones for Rabies and Feline Leukemia (FeLV). These sarcomas have most often occurred on the upper back between the shoulder blades, or on the hip, since this is where both vaccinations and medications have typically been given. ISS tumors usually appear as round, hard lumps underneath the skin. They can develop weeks, months, or even years after a vaccination.

Lowering The Risk Of Injection-site Sarcomas

In 1996, the Vaccine Associated Feline Sarcoma Task Force (VAFSTF) was formed to help formally research the link between vaccination and sarcoma in cats. One of their observations was that the risk of ISS increased whenever multiple vaccinations were given at the same site, which in 1999 led them to recommend making a sweeping change in how veterinarians vaccinate cats.

Until then, veterinarians vaccinated cats with the rabies vaccine in the upper thigh muscle of a back leg, and all other feline vaccines were given between the shoulder blades. However, because an ISS tumor on a cat’s upper back is extremely difficult to completely remove (there often isn’t enough skin left to close the large incision that’s needed), the logic was that if a tumor developed in response to a vaccine on a rear limb instead, if necessary the entire limb could be safely amputated.

Therefore, VAFSTF called for vets to immediately stop vaccinating all cats between the shoulder blades, and instead to vaccinate as follows:

Rabies vaccine: Administered on the right rear leg, as far down as possible.

Other vaccinations: Administered on the right front leg, below the elbow.

This protocol also makes it easier to identify exactly which vaccine caused the tumor if one should form.

It’s now recommended that cats never be vaccinated between the shoulder blades.

Oncologists and many other mainstream veterinarians now also recommend giving vaccines into the cat’s tail. Personally, I am a big proponent of this, and it’s how I prefer to have my own cats vaccinated. If a sarcoma develops, it’s much easier to amputate a tail than an entire leg!

Other Ways To Lower The Risk Of ISS

There are other ways you can work with your veterinarian to help lower the risk of injection-site sarcoma in your cat.

Don’t Over-Vaccinate

It’s now universally understood that vaccines do not need to be given every year. After the initial first series of vaccines, most can be given every 3-5 years thereafter. And not all cats need all vaccines. For instance, if your cat doesn’t go outside and isn’t exposed to unknown cats, in most cases he or she does not need the Feline Leukemia vaccine at all (which carries a higher risk for injection-site sarcoma). Work with your veterinarian to develop a customized protocol for your cat based on his or her level of risk.

Use The Safest Vaccines Possible

Ask your veterinarian about using only non-adjuvanted vaccines, such as the PUREVAX® 3-year rabies vaccine. For kittens, ask your veterinarian to administer the rabies vaccine as close as possible to 6 months of age to lower the potential for a reaction.

Monitor Your Cat Regularly For Lumps And Bumps

Make it a habit to routinely check your cat for lumps and bumps, especially in areas that have been vaccinated. Keep in mind that a small, firm swelling may occur under the skin after a recent vaccination, but this should disappear after 2-3 weeks. If it doesn’t, or if it appears to be getting larger, follow up with your veterinarian.

Treatment and Prognosis For Injection-site Sarcomas In Cats

Injection-site sarcomas are aggressive tumors that can spread very rapidly. They grow deeply into surrounding tissue, making them difficult to remove, and in as many as 60% of cases they tend to recur even after being surgically removed. Up to 25% can also metastasize to other parts of the body, such as the lymph nodes and lungs.

Because of this, treatment is usually done quickly and aggressively. It includes surgical removal of the tumor (with wide margins taken), and radiation and/or chemotherapy to lower the risk of the tumor coming back.

The prognosis for injection-site sarcoma in cats varies from patient to patient. As with most cancers, early detection and quick treatment is key. Tumors that are found when they are still very small are often much easier to remove and treat.

The tail vaccination protocol is highly recommended; in the event of a sarcoma, it’s much better to amputate a tail than a leg.

Weighing The Risks

The diagnosis of an injection-site sarcoma can be devastating, both for the veterinarian and for the cat parent – who despite giving their cat high-quality care by vaccinating them properly, may feel tremendous guilt for inadvertently “causing” their cat’s cancer.

However, deadly diseases occur far more frequently than injection-site sarcomas do, so the risk of ISS is very small when compared to the overall risk of disease. You can lower the risks associated with vaccines by working closely with your veterinarian to develop an appropriate vaccination protocol for your cat. Insist on using only non-adjuvanted vaccines, and don’t over-vaccinate. And if your veterinarian is still vaccinating between the shoulder blades, ask him or her to follow the VAFSTF vaccination protocol or administer vaccines into your cat’s tail instead.

By working together with your veterinarian to decide upon the best vaccination options for your cat, you can rest assured that you’ve done everything possible to minimize the risk of vaccination and to maximize your cat’s chances for a long and happy life with you.

Have you ever had a cat who developed an injection-site sarcoma? Please share your story with us in the comments below!

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Comments

Tuna had a BAD reaction to the rabies vaccine that lasted nearly two weeks and I’m very
hesitant to let him continue to have it; he’s and indoor only cat and I realize the vaccine is
important but I don’t allow him to have it every year ~~~~~~~~ ♥

Oh that is so scary, I’m sorry Tuna! I did some research on this, and found that adverse reactions to the rabies vaccine are more common than originally thought. The American Veterinary Medical Association (AVMA) actually advocates for the use of a Rabies Vaccination Waiver in cases where the vaccine has caused a bad reaction in the past, and it’s believed that future vaccinations may endanger the pet’s life.

According to the AVMA: “AVMA recognizes some animals might require a waiver from mandated rabies vaccination because the vaccination may endanger the animal’s life or a waiver might be necessary for research purposes. If adequate steps can be taken to minimize the chance of exposure to rabies virus, the AVMA recommends that such animals be granted a waiver from mandatory rabies vaccination, upon recommendation of a licensed veterinarian and with the concurrence of the appropriate veterinary public health authorities.” (You can read the complete article here: https://www.avma.org/KB/Policies/Pages/Annual-Rabies-Vaccination-Waiver.aspx )

There are actually 18 states that allow these rabies exemption waivers! Currently, they are: Alabama, California, Colorado, Connecticut, Florida, Illinois, Maine, Massachusetts, Maryland, New Hampshire, New Jersey, Nevada, New York, Oregon, Pennsylvania, Vermont, Virginia, and Wisconsin. (More info here: https://www.rabieschallengefund.org/states-with-medical-exemptions ). If you live in one of these states, you’re in luck! Unfortunately, the remaining states require the rabies vaccine by law and these waivers are not accepted. That being said, in Tuna’s case (where he is an indoor cat and his chance of exposure to rabies is very low), I would ask Tuna’s vet to run a rabies titer test on him every 3 years instead of automatically giving him the vaccine. If his test shows that that he still has sufficient antibodies to rabies, you have a strong case for not having to automatically re-vaccinate him for rabies. Vaccine reactions can be very serious, so I would definitely make that case to both my vet and the department of public health! Good luck Tuna!! 🙂 <3

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I'm an author, founder of the Good Pet Parent Blog, and former Registered Veterinary Technician. Through GoodPetParent.com, I share information on pet health, veterinary terminology, animal behavior and communication, training, and nutrition to help pet parents better understand and care for their animal companions. [More.]